That's a logical explanation. Nonetheless, most business decisions revolve around money. The M lineup is a premium "price" level for 5000 machines. Deere can afford to put the 5-cylinder engine in those tractors and still make money. I have little doubt that the 3-cylinder engine is a lower cost powerplant for Deere. That said, you'd expect that Deere would stand to generate more profit by continuing the 5000E&D build (3-cylinder 179 cu.in.) on up into the M lineup @ 65-75hp.
But... I think the 5-cylinder will outperform an equivalent Hp 3-cylinder engine. And, I believe that Deere expects that people will pay more for that and be more inclined to move up to the M models as a result. Why spend more money for a 5065M-5075M when it's the same "basic" tractor as the 5065E-5075E; if they all had the same 3-cylinder engine? So, Deere went with the 5-cylinder..
Personally, I would have been less hesitant (when I bought my 5075M) if Deere would have offered the lower Hp M tractors with the 4-cylinder 219-239 cu.in. engines. I'm real old-school that way - more cu.in. is always better. Then, when I started digging a bit on the NE test site, I noticed that the 4.5L (276 cu.in.) engine has been used all the way up into the 7000 tier machines!!! They built the 7130 with both a 414 cu.in. powerplant and the 276 cu.in.????
I know which model I'd buy....
However, when I dug around a bit further - it became apparent that Deere has been juggling with engine builds to comply with the emission Tiers. They're working to meet the pollution mandates with the least effects upon fuel economy, reliability and technological capabilities. And of course, money - their costs vs their competitors.
Now, we've got engines with 4 valves per cylinder and variable vane turbochargers, and computer controls that are mind blowing; etc. etc.
AKfish